<em>The question has already been answered, but I guess you need an explanation.</em>
<em></em>
Answer:
<em>They are buying them for him</em>
<em />
Explanation:
To properly understand this, you need to know about the subject-object agreement.
This agreement follows:
[S] [P] [O}
<em>S: Subject represent who does an action</em>
<em>P: Predicate is what is being done</em>
<em>O: Object receives of the action.</em>
<em />
In the given words, the predicate is "buying"
So, you need to know who is buying something and what is being bought
<em>"They" -----> the subject </em>
<em>"are buying" ----> the predicate </em>
<em>"them" ---> the object </em>
<em>"for him" ---> to whom the object is being bought for</em>
